459 
1. Beijing 2008 
APPLICATION OF GPS IN DISTRIBUTED SAR SATELLITE 
Chen Junli a,b *,Zhu jie a 
Shanghai Jiaotong University b Shanghai Institute of Satellite Enginneering 
No. 251,Huaning Rd.,Minhang District, Shanghai, China 
Commission VI, WG 1/2 
KEY WORDS: Distributed SAR Satellite, DEM, GPS, INSAR, GMTI , Baseline Estimation, Time Synchronization, Frequency 
Synchronization 
ABSTRACT: 
In this paper, we proposed a new Distributed SAR Satellite(DSS) using one passive satellite with a dual receive antenna(DRA) 
flying in formation with an already existing conventional SAR satellite. Such system can provide either GMTI or INSAR functions 
or both. However, the complexities of DSS imaging increase due to the synchronization problems and Baseline estimation problem. 
Accuracy requirement for baseline estimation and synchronization are given. A method based on dual frequency GPS is introduced 
to solve the Baseline estimation and time & frequency synchronization problems of Distributed SAR Satellite. 
1. INTRODUCTION 
In many civilian and military applications of spacebome SAR 
imaging, it is highly desirable to simultaneously monitor 
ground traffic and acquire a global DEM. Distributed SAR 
Satellite(DSS) operate with multiple receive antennas which are 
mounted on different satellites(Massonnet,2001). Powerful 
applications of DSS are single-pass cross-track 
interferometry(XTI) and along-track interferometry (ATI). The 
satellite orbits are designed such that the constellation provides 
a vertical and a horizontal baseline at the same 
time(Meyer,2005). So, this concept can be used for DEM and 
GMTI. 
The DSS consists of master satellite with radar transmitter and 
receiver and one slave satellite with only receivers. Time & 
frequency synchronization error occurs because of the different 
formation flying platforms and different frequency sources. In 
INSAR processing the knowledge of accurate baseline length 
and orientation is important for DEM accuracy. The 
requirement of error of baseline measurement for creating 
accurate DEM is very stringent. Baseline estimation and Time 
& frequency synchronization are three key technologies for 
DSS(Matthias,2004). 
2. SYSTEM DESCRIPTION 
2.1 System Overview 
In this paper, we considered the problem of Spacebome 
INSAR and GMTI system and proposed a distributed satellite 
constellation using one passive satellite with a dual receive 
antenna(DRA) flying in formation with an already existing 
conventional SAR satellite. This novel proposed configuration 
operates in a 3 channel mode, thus can provide a combined 
XTI/ATI acquisition with across-track and long& short along- 
track separation (see figure. 1). INSAR, or ATI, makes use of 
phase difference to measure topography or GMTI. Such system 
can provide either GMTI or INSAR functions or both. Large 
along-track baselines are required for accurate measurements of 
slow movements, while short baselines are required to avoid 
ambiguities in case of higher velocities, thereby enabling 
improved and more accurate measurements over a wide 
spectrum of potential scatterer velocities. The combination of 
the different along-track baselines will be used for improved 
detection, localisation and ambiguity resolution in ground 
moving target indication (GMTI) and traffic monitoring 
applications. 
This approach is especially attractive, since it will enable a cost 
efficient and easy implementation for spacebome sensor. Such 
system can provide either GMTI or INSAR functions or both, it 
has two acquisition modes: (1) Tri-baseline ATI mode,(2) 
Simultaneous INSAR and ATI mode. 
2.2 Tri-baseline ATI performance 
The basis of the ATI technique is that the interferometric 
combination of two complex SAR images of the same scene, 
acquired with a short time lag, is sensitive to the Doppler shift 
from the line-of-sight velocity of targets. The three SAR image 
* Corresponding author.